The optional picture is searched in the picture
directory you determined in the configuration file
config.spip.
Sometimes, if you use special HTML tags inside your data tag,
you have to add the line <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
as the second line of your sheet file.